1. Field of the Invention
The present invention relates to a sensor element, especially for pressure sensors, comprising a first membrane mounted in a rigid frame, said membrane being in pressure communication with two volumes between which a pressure difference is to be measured.
2. Description of the Related Art
Pressure sensors are increasingly more popular for different purposes, from measuring pressure in car tires to pressure measurements during drilling operations at sea. Pressure sensors are in increasing degree made from semiconductor materials, such as silicon, using techniques for micro machining.
The pressure sensors usually consists of a membrane or similar between two volumes between which the pressure difference is to be measured, one of the volumes usually having a reference pressure or possibly a slowly changing pressure.
At pressure differences the membrane is bent, at least along the edges. The bending is measured using strain sensitive piezoelectric elements being positioned in the areas of the membrane being deformed.
There exists a number of different types of such sensors, e.g. as described in the German patents DD 267,107, DD 286,222 and DD 291,398, as well as U.S. Pat. No. 5,174,690. In a number of these publications the sensors are coupled to each others in pairs, so that one is mounted in parallel with the implied strain and the second is mounted perpendicular to this direction. Through a bridge coupling the changes in the characteristics of the sensors, e.g. resistivity, are measured indicating the degree of deformation and thus the pressure.
A problem related to this type of pressure sensors is that they have a limited measuring range. When the pressure (or pressure difference) exceeds a critical value the bending strain in the edge of the membrane will result in breaking of the membrane.
A way to avoid this is to stop the membrane when a certain pressure is exceeded, e.g. by providing a support surface, as shown in DD 291,398. This will, however often reduce the sensitivity of the membrane and increase its complexity.
In the present invention the problem is solved in a different way, as the area being subject to bending strain is moved from the soft to the rigid membrane as the pressure increases. since the material withstands stretch better than bending the membrane will not break. By also measuring the bend in the stiff membrane an extended measuring range is obtained compared to measurements performed with a conventional membrane.
It is an object of this invention to provide a sensor element which may be produced with ordinary production methods and which has an extended sensitivity range relative to the known solutions. It is also an object of this invention to provide a sensor element with a simplified overload protection. To obtain these goals a sensor element is provided as described above being characterized as described in claim 1.